For me there's a difference between fastest/slowest and most comfortable, so my answer might not satisfy you 100% but here it is anyway.
Let me start by mentioning two grounds I don't like:
I don't like sand. It's coarse and rough and irritating and it gets everywhere.
Snow is too cold for my liking, I always have to put fur boots when I'm walking on snow and fur boots ain't the most fashionable boots ever if you ask me...
Not to mention ice, it's not only cold but also slippery...
My favorite ground (ekhm... "ground") to walk on are tiled roofs. The higher the roof, the better. Roofs provide great views and even greater solitude. Everyone needs solitude and beatiful views from time to time. But what I love the most about those roofs is the fact they are so safe! You cannot fall from them!